Security and the Management Plane, Part 1

In part 1 of a two-part article, network management specialist Stephen Morris discusses the management plane, its interaction with the other planes, and the need to maintain its security. Some important principles of network management are also discussed along the way.

From the author of

Overview of the Management Plane

A few years ago I overheard an IT staffer saying, "I know the salaries
of all the people on this site. They're stored on a remote disk and I have
the password." This security incident illustrates the crucial point that up
to 80% [1] of attacks originate from within organizational boundaries. We should
always bear this principle in mind when thinking about securing any system or
network:

Security is only as strong as its weakest link (often human or
process).

So what's so special about the management plane?

The management plane (MP) provides an important element in the orderly
running of networks. It's a valuable resource for deploying and managing
increasingly advanced network services including (among others):

IP-based differentiated servicesvoice/video-over IP

Layer 3 (or IP) VPNincorporating MPLS

Layer 2 VPNextending the reach of a LAN across a service
provider

The demand for these services is growing and many service providers are
deploying them. This change is in response to both user demand and the threat to
service providers of price-sensitive, commoditized business models. The
service-level agreement (SLA) is a key commercial differentiator of the above
services, but SLAs are difficult to achieve without leveraging the MP.

A further, more subtle role for the MP is enabling the alignment of IT and
enterprise core business processes. While not the subject of this article, this
important topic is all the rage nowadays. One way to effect this alignment is
for IT to explicitly define and document the services it manages within an
organization, such as the following:

Client and server machine configuration

Segmenting the network into virtual LANs (VLANs)

Software distribution and license management

Network protection (antivirus) and security

In a separate step, these services are typically rolled out onto the network
via the MP of the associated devices. Following this point, the MP is used to
keep the services up and running.

If the process of service definition, rollout, and subsequent management is
automated, this setup becomes a powerful way for IT to run the network. In turn,
this power can be leveraged by the organization to help achieve its business
goals; for example, extending LAN-level services to remote workers, creating
secure extranets, and so on. Unfortunately, most organizations use scripts and
other proprietary facilities to run their networks. This technique tends to
restrict the potential for aligning IT and business goals because so much IT
staff time is spent firefighting.

Automating MP interactions (for instance, using network management software)
facilitates powerful business-enabling processes. In many ways, the industry
focuses on the control plane, often at the expense of the MP. In this article
and the next, I hope to illustrate the point that the MP is often an
underutilized resource. Hence, the need to secure it!

Security Threats

Traditional threats to security include denial-of-service, hacking, snooping,
data modification, and more. The increased deployment of differentiated IP-based
services provides the opportunity for a theft-of-service type of attack. In this
attack, a user fraudulently marks IP packets in order to receive a higher level
of service.

The MP provides facilities for helping in the war against these and many
other types of attacks. Let's take a closer look at the MP.

The Management Plane: Network Gold

The MP performs management functions for an overall system and provides
coordination among all the planes (control, data, forwarding, and so forth). The
MP, control plane, and data plane are illustrated in Figure 1 in conjunction
with a remote management system.

The management system in Figure 1 may be one or more of the following:

Element Management System (EMS). Handles one or more devices and
provides a device-centered perspective.

Network Management System (NMS). Handles many devices and provides
a network-wide perspective (often communicates with the EMS).

Operational Support System (OSS). Handles network-wide operations
with interfaces to upper business layers (often drives the NMS).

Regardless of the management system type, all device access passes through
the MP. In Figure 1, the MP is shown interacting with the control plane (such as
changing a routing table entry on one of the devices in the managed
network).

Under the direction of the network manager, the management system interacts
with the MP, typically performing reads and writes of managed objects. The MP
can also relay notification messages that are sent autonomously from the network
to the management system. Typically, the latter message type signifies some
important event or fault that has occurred in the network, such as these:

Onset of congestion in some important part of the network (may require
reaction within a few hours)

Completion of a software upgrade on a node (may be purely
informational)

The bidirectional message exchange allows the management system to keep track
of and modify the state of the underlying managed network devices. The MP
facilitates this process and allows the management system (and hence the network
manager) to keep track of the picture it has of the network.

NOTE

A fundamental rule of network management is that the management system is
constantly working to maintain an accurate picture of the state of the managed
devices. [2] The more closely matched the management system picture is with the
real network, the better.

An important point about Figure 1 is that each managed devicerouter,
switch, hub, etc.contains its own set of planes (not shown in Figure 1).
It is, however, a major goal of the management system (particularly the NMS and
OSS) to provide a single system image of the underlying network. In this
context, the three planes in Figure 1 can be considered to represent an entire
network of devices. This useful abstraction facilitates management scalability
as the network grows.

As we'll see later, security demands that every element of Figure 1 be
individually secured.

NOTE

It's axiomatic to say that a network is generally only as fast as its
slowest element. Similarly, a network is generally only as secure as its least
secure element.

Why Provide Planes?

Wouldn't it be easier if networks were just monolithic entities, without
having to think about all these planes? Unfortunately not, particularly given
the growing complexity of network devices. The provision of planes greatly
assists pretty much everyone in the networking value chain: developers, testers,
installers, end users. To understand why, let's look at a few common
characteristics of network devices:

Expensive to develop and purchase

Long lived, they may reside in managed networks for years

Upgraded over time to add new technologies, such as MPLS, layer 2/3
VPN

Try to think of network devices (routers, switches, hubs, etc.) as
special-purpose computers. Typically, they contain many different interfaces on
which traffic is sent and received. Modern devices are equipped with interfaces
that support many technologies, such as these:

Ethernetlegacy or VLAN-aware or QoS-aware (i.e., IEEE
802.1p/Q)

HDLC/PPP

Frame relay

ATM

MPLS

IP

Configuring and managing services on any of the above is decidedly
non-trivial! Many different elements have to be configured, such as layer 2/3
addresses, subnet masks, routing/signaling protocols, virtual circuits, and
more.

Separating the various areas of device management, control, and data (among
others) into individual planes [3] provides a useful abstraction. End users can
then become accustomed to a stable MP user interface; for example, the Cisco CLI
is a de facto standard. SNMP provides a slightly less visible point of MP access
but is a (de jure) standard nonetheless. By placing CLI and other access
technologies into the MP, device vendors can provide a useful division of labor.
Unfortunately, sometimes the division into planes results in MP development
taking place late in the project lifecycle. This delay presents its own special
problems!

If a vendor wants to add new technology support to a device, the vendor
(initially) only has to upgrade the control plane. Associated changes to the
other planes can then be applied later, rather than having to update all planes
at once. This stratagem makes development somewhat easier to control and is
similar in concept to the layered architecture of network protocols.

Accessing the Management Plane

Traditionally, access to the MP is made using either the command-line
interface (CLI) or a network management protocol (such as SNMP or OSI).
Management access to the MP of a given device typically occurs through a
dedicated interface such as an Ethernet port, serial port, and so on. Figure 2
illustrates a hard-pressed network manager laboring to keep a big network up and
running while her coffee goes cold. Also in Figure 2, we see the two access
methods (CLI/SNMPv3) in use by the NMS. We assume that SNMPv3 is in use on all
devices in the network. This may not be the case in practice; e.g., legacy
devices may not support SNMPv3.

Many network devices also support web accessboth secured and unsecured.
Many vendors support CLI, SNMP, and web-based access, essentially providing the
same facilities from all three methods. In this article and the next, I'll
focus on CLI and SNMP only.

CLI is little more than a text menu-based interface, much like that of
MS-DOS. Some vendors provide very fine-grained control as part of CLI; for
example, with numerous levels of access.

SNMP is somewhat more sophisticated than CLI because it's a standard,
message-based protocol with a well-defined, extensible data model.

NOTE

The difference between CLI and SNMP is that CLI provides a human-to-machine
interface, while SNMP uses a machine-to-machine interface.

It's likely that as technology advances, the use of SNMP (in conjunction
with NMS products) will overtake CLI.